For almost all Tier 2 Licence applications the UKBA requires a minimum of four pieces of supporting evidence. Some of these pieces of evidence are mandatory, others are applicable to certain types of business, and the remainder can be chosen from a list of optional documents.
But, BEWARE, and read the small print. In some cases a single ‘piece of evidence’ may comprise more than one document. A classic example – you may need to provide a VAT certificate AND your most recent VAT return. Or a bank statement AND a letter from your bank confirming the history of your account. Submitting only item from such a pair of documents won’t help you at all – it won’t count as a valid piece of evidence. And two ‘halves’ from different pieces of evidence won’t make a valid whole either, for example you can’t submit a bank statement and a VAT certificate and hope that will count.